Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why the moon looks bigger at the horizon

  1. Aug 31, 2007 #1

    daniel_i_l

    User Avatar
    Gold Member

    For a while I've been wondering why the moon looks so big on the horizon. I looked around the web and found 2 answers:
    1) At the horizon the brain thinks that it's very far way compared to when it's above your head in the sky, but since they're really the same size in both places the brain corrects for this and assumes that the moon near the horizon is bigger.
    2) When the moon is low down you have a lot of things on the ground to compare it with and it looks big relative to them. But when it's high in the sky you have nothing to compare it with.
    Is there any way to check which explanation is correct?
    And is it really just an illusion with no optical explanation?
    Thanks.
     
  2. jcsd
  3. Aug 31, 2007 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    It's an optical illusion. A very, very good one, too. The two answers say more-or-less the same thing. What's the conflict? The second is just a bit more descriptive.
     
  4. Aug 31, 2007 #3

    DaveC426913

    User Avatar
    Gold Member

    1) is the answer I have heard too.

    Our perception of the sky - in the absence of visual clues - is that of a low dome, its roof closer to us than its rim. Thus, when we see the Moon above us, we perceive it to be closer than when it is at the horizon. Because we think it's closer yet its disk is the same size, we interpret that as if it is smaller over our heads.

    Fig 2 on this page explains it.

    [ EDIT ] Huh, it goes on to explain an effect I had not thought of. Clouds over your head are in fact much closer than clouds over the horizon. This lends plausibility to the idea that our brains might perceive the sky to be closer above us than near the horizon - since that's actually true. It is only necessary to suppose that the effect remains part of our expectation even when there are no clouds in the sky.
     
    Last edited: Aug 31, 2007
  5. Aug 31, 2007 #4

    DaveC426913

    User Avatar
    Gold Member

    Well, the second one doesn't actually explain the effect; it simply hand-waves it. So, it's not that 1) and 2) are the same, its that 2) says nothing of value.
     
  6. Aug 31, 2007 #5

    DaveC426913

    User Avatar
    Gold Member

    Also interesting:

    "My personal contribution to the issue (and my only published contribution to the science of astronomy, as printed in Sky & Telescope, May, 1985, letters) is the following observation: use a tube over one eye to eliminate the foreground (close the other eye); note how small the moon suddenly appears. Now open the other eye. The eye with the unobstructed view sees a larger moon, and you have the slightly unnerving experience of seeing two different-sized moons at once. This observation proves that the illusion occurs independently in each eye, and places limits on any mechanism proposed to eplain it."
     
    Last edited: Aug 31, 2007
  7. Aug 31, 2007 #6

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    I saw (1) as being pure hand-waving and saying nothing of value while (2) gives some clues regarding the brain does what it does..
     
  8. Aug 31, 2007 #7

    rcgldr

    User Avatar
    Homework Helper

    I tried hand-waving while looking at the moon and it made no difference. Will have to go with one of the previously posted reasons, or with the reasoning that at the horizon, there are other objects to compare the moon with (for apparent distance from the viewer), while once it's up high enough a direct comparason can't be made (can't tell if the moon is in front of or behind other objects unless there are clouds).
     
  9. Sep 1, 2007 #8

    daniel_i_l

    User Avatar
    Gold Member

    Thanks for the replys! I can't wait untill the next time I see it so that I can try what Dave posted with the tube.
     
  10. Sep 1, 2007 #9
    As I recall, this has been tested in planetariums, and sure enough, if you cast a photo of the moon horizontally, people will almost always think it's bigger than if you cast it straight up.
     
  11. Sep 3, 2007 #10
    A related phenomena I find when I look at the moon is that when the dark side of the moon is more visible (i.e. when you get a more three dimensional sense of the moon) the moon looks bigger (in addition, you really get the sense of «hey, there's a goddamn gigantic sphere up there!»).
     
  12. Sep 5, 2007 #11

    LURCH

    User Avatar
    Science Advisor

    I can't agree with answer #2, since the Moon looks bigger when it's at the horizon over perfectly smooth water, when there are no objects with which to compare it. I also can't agree that the two statements are saying the same thing.

    Therefore, I tend to accept the first explanation, that our mind uses the horizon as a marker for being really far away, and anything beyond it is perceived as being very distant.
     
  13. Sep 6, 2007 #12
    I also read that in the brains eye, the earth is dome shaped so anything appearing closer to the horizon seems larger when compared to being perpendicular to the observer. I guess I would have to agree with explanation number one.
     
  14. Sep 14, 2010 #13
    I think nobody has said the right thing so far. Even you lie down on the ground to look into the moon that is in the middle of the sky dome, it is also small.
    the real science behind this phenomina is that the Earth has a transparent atomosphere acting like an optical lens causing telescoping effect. only when the moon or sun on the horizon is a convex lens standing between you and the moon or sun, because the atomosphere is a transparant round ball suround the earth. The light beam has different behaviour in the air than in the vacuum outside the atomasphere. Asume the atomsphere ball are water or glass, you will sure understand what I am saying. When the moon is right on the sky dome, light beam goes to you without bending toward the center of the sphere because it is purpendicular to the surface of the spheric convex lens. but when it is on the horizon, your are at one tip of the lens because the center of the lens is the center of the earth, you are too far away from the center because you are on the earth surface. We sure know the moon did not change size, but the light beam from it changed direction to reach our eye causing a bigger visual angle so it looks larger. Our brain will not feel anything bigger if the visual angle doesn't increase. It is not an illusion. We have a brain but a real camera hasn't, the real camera get a big sun at suset, do you think the camera had an illusion????
     
    Last edited by a moderator: Sep 14, 2010
  15. Sep 14, 2010 #14
  16. Sep 14, 2010 #15
    Well that doesn't really make sense, because imagine you were a person in the time before they knew clouds were closer than the moon: you'd still perceive this optical illusion; so it doesn't seem to be contributing at all.

    EDIT: lol just saw the previous post and its edit (wasn't there when I posted)
     
  17. Sep 14, 2010 #16

    DaveC426913

    User Avatar
    Gold Member

    Your facts about the atmosphere, while true, are a red herring. They lead you to the wrong conclusion about the Moon illusion.

    It is an illusion.
    A camera is how you prove it's an illusion. A camera is not fooled.

    The moon does not change size as it nears the horizon.

    Take as many pictures as like, they will show the same thing. (The critical thing to ensure though, is that the images of the Moon high and Moon low must be in the same picture - i.e. with the same camera settings - for comparison.)
    [​IMG]
     
    Last edited: Sep 14, 2010
  18. Sep 14, 2010 #17
    You should also try this: after observing it at the horizon normally, bend over and look at it from between your legs. It will look normal size again. It has everything to do with our perception that things that are nearer are larger. I was just reading about this effect the other day, in fact: http://www.michaelbach.de/ot/sze_moon/index.html
     
  19. Sep 14, 2010 #18

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    A real camera does not get a big celestial object (sun, moon, or even a constellation) when the object is at the horizon. There is an atmospheric effect, but it is rather small and it opposes the illusion. Objects near the horizon have a slightly smaller angular size due to atmospheric effects.
     
  20. Sep 14, 2010 #19

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Has anyone ever seen the full 'harvest' moon rising over the Atlantic? It is orange, and it is HUGE...appearing much larger than an ordinary full moon rising over the Atlantic horizon, and appearing much much larger than a moon overhead. Now the moon doesn't change size, and it doesn't have anything to do with the apogeee or perogee. It's got to be more than an illusion, it must be due to the length of sight through the atmosphere when looking toward the horizon, and atmospheric conditions in particular at the autumn equinox when the Harvest moon appears.

    Watch for it this month on the 22nd I think, and you'll see for yourself!:bugeye:

    Note: I'll be out there with a toilet paper roll peering through from one eye, hoping that the neighbors aren't watching!:tongue:
     
  21. Sep 14, 2010 #20

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    While the atmosphere does have some effect on the Moon near the horizon, it isn't what you think it is. For one, the index of refraction for air is pretty small (1.0003), For the Other the effect it does have is to bend light slightly around the horizon, so that we see objects that, in a straight line, are below the horizon and objects just at the horizon are seen higher in the sky. This bending effect gets stronger as your line of sight nears the horizon. When the Moon is sitting right at the Horizon, the effect is stronger at the bottom of the Moon than it is at the top, so the actual effect is to "flatten" the Moon slightly, not enlarge it.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Why the moon looks bigger at the horizon
Loading...